The present invention relates to a method for absolute asymmetric synthesis. More specifically, the invention relates to a novel method for absolute asymmetric synthesis, which enables concentration of one of the enantiomers or diastereomers in the starting material and one of the enantiomers or diastereomers in the product, in a reaction system in which the enantiomers or diastereomers are not converted into each other chemically or thermally.
Asymmetric synthesis plays an important role in the preparation of various optically active chemicals, such as pharmaceuticals, agricultural chemicals, perfumes, cosmetics, and intermediates thereof. In the second half of the nineteenth century, the usefulness of right- and left-circularly polarized light (r-CPL and l-CPL) was confirmed by van""t Hoff. Since then, there have been many attempts to produce optically active chemicals using circularly polarized light (CPL).
Such synthesis of optically active chemicals using CPL is one type of xe2x80x9cabsolute asymmetric synthesisxe2x80x9d (AAS). That is, asymmetric induction is realized by preferential excitation of one of the enantiomers by means of irradiation with right- or left-circularly polarized light. The anisotropic factor, also known as xe2x80x9cgxe2x80x9d, is considered to determine the degree of selective excitation. The anisotropic factor g was defined by Kuhn as the difference between optical isomers in molar absorption coefficient for r- or l-CPL at a certain wavelength as follows:
g=(xcex51xe2x88x92xcex5r)/xcex5=xcex94xcex5/xcex5xe2x80x83xe2x80x83(1) 
in which xcex5=(xcex51xe2x88x92xcex5r)/2 and Oxe2x89xa6g less than 2 (Trans. Faraday. Soc. 1930, 293-309; Z. Phys. Chem., B. Abt. 1930, 7, 292-310).
Absolute asymmetric synthesis (AAS) is classified into three categories: (a) asymmetric photolysis, (b) photochemical deracemization and (c) photochemical asymmetric fixation.
In asymmetric photolysis, the two enantiomers in the starting material are photochemically decomposed in different extents depending on the degree of their selective excitation by irradiation with r- and l-CPL at a certain wavelength. Scheme (1 shows this asymmetric photolysis. 
In place of xcex5l and xcex5r, xcex5R and xcex5S are used herein. xcex5R and xcex5S represent molar absorption coefficients of the two enantiomers for r-CPL or l-CPL, respectively. The enantiomer less excited by r- or l-CPL remains in the starting material and the optical purity is increased, whereas the other enantiomer decomposes. In this type of absolute asymmetric synthesis, the photochemical process is irreversible. There are many reports on such asymmetric photolysis. Representative examples are a report on photolysis of camphor (Z. Phys. Chem., Abt. B, 1930, 292-310) and a report on photolysis of trans-bicyclo[4,3,2]nonan-8-one (J. Chem. Soc., Chem. Commun. 1978, 983-4).
Scheme [2] shows a photochemical deracemization process. As shown in Scheme [2], the total concentration of the enantiomers does not change during the photoreaction. Preferential excitation of one of the enantiomers over the other shifts the photochemical equilibrium, and the enantiomer ratio is fixed upon termination of irradiation. However, except for inorganic compounds (Mol. Photochem. 1969, 1, 271; Chem. Commun. 1996, 2627-2628), there are only a few reports of photochemical deracemization. Organic compounds, which exclusively undergo photoderacemization, have been rarely reported. Actually, it appears that side reactions always occur when organic compounds are photochemically deracemized. 
The photochemical process of asymmetric fixation resembles asymmetric photolysis. In the process, the starting material undergoes thermal racemization and as shown in Scheme [3], an enantiomer-selective photoreaction is induced by irradiation with r- or l-CPL and thereafter an optically active product is obtained. The R/S ratio of the product is equal to the molar absorption coefficient ratio, xcex5R/xcex5S. There are not many examples of such photochemical asymmetric fixation. This type of photochemical asymmetric fixation includes, for example, oxidative photocyclization of 1-(2-benzo[c]phenanthryl)-2-phenylethylene to hexahelicene via dihydrohericene (J. Am. Chem. Soc. 1971, 93, 2553; J. Am. Chem. Soc. 1973, 95, 527-32). 
The reversible absolute asymmetric synthesis of 1,1xe2x80x2-binaphthyl pyran recently proposed by G. B. Schuster, et al. is a variation of photochemical deracemization (J. Am. Chem. Soc. 1998, 120, 12619-12625).
As shown above, conventional methods for absolute asymmetric synthesis using circularly polarized light are methods for obtaining, in the starting material, an excess amount of one enantiomer relative to the other primarily by utilizing preferential decomposition or shifting the enantiomer ratio by means of irradiation with circularly polarized light.
Under the circumstances described above, the present invention was made. The present invention focuses on the reaction product to which none of the conventional researchers have paid attention. An object of the present invention is to provide a novel method for absolute asymmetric synthesis, which enables concentration of one of the enantiomers or diastereomers in the starting material and one of the enantiomers or diastereomers in the product, in a reaction system in which the enantiomers or diastereomers (i.e., R-isomer and S-isomer relative to one asymmetric carbon) are not chemically or thermally converted into each other.
To achieve the above object, the present invention provides the following inventions:
First, the present invention provides a method for synthesizing absolute asymmetry which comprises: providing a photochemically reversible reaction system in which the starting material is a mixture of enantiomers or diastereomers not photochemically or thermally converted into each other; and irradiating the reaction system with right- or left-circularly polarized light to excite the starting material alone or both the starting material and the product, thereby concentrating one of the enantiomers or diastereomers in the starting material and one of the enantiomers or diastereomers in the product that corresponds to the enantiomer or diastereomer not concentrated in the starting material.
Secondly, the present invention provides the above method in which the starting material and the product are mixtures of enantiomers and only the starting material is excited, the concentration of one of the enantiomers in the starting material and one of the enantiomers in the product being controlled by adjusting the anisotropic factor g, which indicates the degree of selective excitation by right- and left-circularly polarized light.
Thirdly, the present invention provides the above method in which the starting material and the product are mixtures of enantiomers and both the starting material and the product are excited, the concentration of one of the enantiomers in the starting material and one of the enantiomers in the product being controlled by adjusting at least one of the following:
the value of anisotropic factor g which indicates the degree of selective excitation by right- and left-circularly polarized light;
plus or minus sign of g; and
K indicating the photochemical equilibrium of the reaction.